Within the project on the webinterface you will also find the command line instructions.

Accepting new SSH key (only for some users with existing repository)

Some users will see the following message when they want to make a remote connection with SSH. (existing repository)
When you see the below message you will need to accept the new key.

$ Warning: the ECDSA host key for 'smlgitlab.socsci.ru.nl' differs from the key for the IP address 'x.x.x.x'
$ Offending key for IP in /home/user/.ssh/known_hosts:23 Matching host key in /home/user/.ssh/known_hosts:13
$ Are you sure you want to continue connecting (yes/no)? yes

All should be fine when the new key got accepted.
Remote SSH connection should be established.